A 12 year old negro girl was studied who presented a picture of megaloblastic anemia refractory to physiologic doses of vitamin B-12 and to folic acid. However, remission could be induced and maintained by pharmacologic doses of vitamin B-12 (1000 µg. per week), by folinic acid or by a combination of vitamin C (or homocysteine) and folic acid. Folinic acid determinations and the clinical response to various metabolites indicated the existence of a metabolic defect in the utilization of folinic acid. The hereditary nature of this defect was suggested by the fact that two siblings might have had a similar condition.